1. Field of the Invention
The present invention relates to an anti-skid control system for use in an automotive vehicle, and more particularly to an anti-skid control system for controlling braking force applied to road wheels in braking operation to prevent the road wheels from being locked.
2. Description of the Prior Art
It is known that the vehicle stability or the controllability is influenced detrimentally depending upon the road surface condition, when road wheels are locked in abrupt braking operation. In order to prevent the road wheels from being locked, therefore, there has been employed an anti-skid control system which controls the braking force by decreasing, increasing, or holding a hydraulic braking pressure supplied to a wheel brake cylinder, and which is also called as an anti-locking control system. In view of the fact that when the hydraulic braking pressure supplied to the wheel brake cylinder is increased, the rotational speed of the road wheel is rapidly reduced immediately before a coefficient of friction of a road surface relative to the road wheel reaches a maximum, the anti-skid control system controls the wheel cylinder pressure according to the deceleration of the vehicle in order that a slip rate of the road wheel results in around 20%, that is, the maximum coefficient of friction is obtained.
As for the control of the braking force applied to the road wheel according to the above anti-skid control system, if the braking force is applied to a pair of right and left road wheels when the vehicle is running on a road surface having a different coefficient of friction between the surfaces at each side of the vehicle in the moving direction thereof, or right and left road wheels, that is, when the vehicle is running on a non-symmetrical road surface or so-called split road surface, as in the case where one of the road wheels at either side of the vehicle is on the road surface having the lower coefficient of friction, and the other is on the road surface having the higher coefficient of friction, an usual anti-skid control will be insufficient, and further a steering control for compensating a yaw moment may be necessitated. For example, if the anti-skid control is initiated when a road wheel at one side of the vehicle is running on a dry road surface, and a road wheel at the other side of the vehicle is running on a wet road surface, and if the braking force is controlled individually depending upon the locking condition of each road wheel during the anti-skid operation, the road wheel on the road surface of the lower coefficient of friction such as the wet road surface will tend to be locked immediately so that the anti-skid control will be initiated, whereas the road wheel on the road surface of the higher coefficient of friction such as the dry road surface will not tend to be locked immediately so that a sufficient braking force will be applied. Thus, the yaw moment will arise when the vehicle is running on the split road surface, notwithstanding that the vehicle is equipped with the anti-skid apparatus, and the yaw moment will force the vehicle to swerve toward the side of the road surface having the higher coefficient of friction, so that the driver of the vehicle will have to countersteer immediately.
In Japanese Patent Publication No. 56-28738, which corresponds to U.S. Pat. No. 3,840,278, it has been proposed that a common pulse source is provided for both brakes located at opposite sides of the vehicle and the pulse source is controlled by two acceleration sensors for sensing wheel acceleration at opposite sides of the vehicle, whereby the wheel braking pressure is applied to one side of the vehicle only slowly, even if only a wheel on the other side has begun to lock, and at the side of the vehicle where better road conditions prevail, e.g. a dry side of the road, braking pressure will increase at a lesser rate than theoretically permissible. Then, it is concluded that the yaw moment will increase slowly, easily permitting counter steering. Also, in Japanese Publication No. 59-19863, which corresponds to U.S. Pat. No. 3,918,766, it has been proposed to cause a brake pressure on a first vehicle wheel having a higher road adhesion than a second wheel on the side of a vehicle opposite the first wheel to assume a holding phase when a skidding tendency of the second wheel is indicated, and during this holding phase, the first wheel brake pressure is held constant at a level attained at the time the skidding tendency of the second wheel is indicated. Then, it is concluded that less initial steering response to the resulting vehicle yaw is required and consequently less oversteering correction and oscillation results.
However, either system disclosed in the above publications will cause a stopping distance of the vehicle to be increased. In the case where the vehicle is driven at high speed, the difference between the braking pressures in the wheel brake cylinders for right and left road wheels must be controlled less than a certain value in order to maintain the controllability of the steering, since the larger the difference is, the more the steering must be steered. Accordingly, supposing that the condition for the anti-skid control is set with its upper limit of the difference between the hydraulic braking pressures in the wheel brake cylinders mounted on the right and left road wheels on the road surfaces having a different coefficient of friction when running at high speed, in the case where the hydraulic braking pressure is controlled when running at low speed, the hydraulic braking pressure applied to the road wheel on the road surface of the higher coefficient of friction is gradually increased even if the steering may be steered appropriately. This will cause the stopping distance to be increased.